PURPOSE: To make round-trip travel and turning steerage of a car body simply and accurately performable by installing a rear-wheel steering wheel for rear wheels at the side of a driver's seat in addition to a front-wheel steering wheel for front wheels.

CONSTITUTION: A tractor body 6 is provided with each of a symmetrical pair of front wheels 1 and rear wheels 4, while these wheels 1, 4, for example, the whole part is formed into a 4WD system being mounted in the front and performing power transmission from an engine 7. In this case, a front-wheel steering wheel 2 for the front wheels 1 is installed in the front side of a driver's seat 3, while a rear-wheel steering wheel 5 is installed in the lateral side of the driver's seat 3. For example, the front wheels 1 are steered by the front- wheel steering wheel 2 via a steering wheel shaft 8, a steering shaft 9, a pitman arm 10, a steering rod 11 and a steering arm 12 or the like. In addition, the rear wheels 4 are steered by the rear-wheel steering wheel 5 via each of identical means 13-16.